The patient with a normal immune system who is given blood transfusions has a considerable probability of developing cytomegalovirus (CMV) infection; the risk ranging from 7% for the recipient of a single unit of blood to three times that number when the patient receives multiple transfusions. Cytomegalovirus infection occurs in over half the individuals given renal allografts and treated with various immunosuppressive regimens. When clinical illness accompanies CMV infection in either of these groups of patients, those with normal or depressed immune systems, the symptom complex may vary widely. The commonly recognized syndromes include "mononucleosis with negative heterophile" or a severe interstitial pneumonia. Laboratory studies support a causative role for CMV in many cases of the "post-transfusion" or "post-transplantation" syndrome but the source of the CMV virus, whether it is endogenous or exogenous and the pathogenesis of these syndromes has not been elucidated. It is our objective to investigate the incidence, associated causative factors and clinical characteristics of CMV infections in renal allograft recipients and to pursue studies of pathogenesis in the monkey. In addition, we propose to evaluate methods which may be useful for the identification of the potential blood donor who has CMV present in his blood.